Silver halide-containing photothermographic imaging materials (i.e., heat-developable photographic elements) processed with heat, and without liquid development, have been known in the art for many years. These materials are also known as "dry silver" compositions or emulsions and generally comprise a support having coated thereon: (a) a photosensitive compound that generates silver atoms when irradiated; (b) a relatively or completely non-photosensitive, reducible silver source; (c) a reducing agent (i.e., a developer) for silver ion, for example the silver ion in the non-photosensitive, reducible silver source; and (d) a binder.
In photothermographic emulsions, the photosensitive compound is generally photosensitive silver halide which must be in catalytic proximity to the non-photosensitive, reducible silver source. Catalytic proximity requires an intimate physical association of these two materials so that when silver atoms (also known as silver specks, clusters, or nuclei) are generated by irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the reducible silver source within a catalytic sphere of influence around the silver specs. It has long been understood that silver atoms (Ag.degree.) are a catalyst for the reduction of silver ions, and that the photosensitive silver halide can be placed into catalytic proximity with the non-photosensitive, reducible silver source in a number of different fashions. The silver halide may be made "in situ," for example by adding a halogen-containing source to the reducible silver source to achieve partial metathesis (see, for example, U.S. Pat. No. 3,457,075); or by coprecipitation of silver halide and the reducible silver source (see, for example, U.S. Pat. No. 3,839,049). The silver halide may also be pre-formed (i.e., made "ex situ") and added to the organic silver salt. The addition of silver halide grains to photothermographic materials is described in Research Disclosure, June 1978, Item No. 17029. The reducible silver source may also be generated in the presence of these ex situ, pre-formed silver halide grains. It is reported in the art that when silver halide is made ex situ, one has the possibility of controlling the composition and size of the grains much more precisely, so that one can impart more specific properties to the photothermographic element and can do so much more consistently than with the in situ technique.
The non-photosensitive, reducible silver source is a material that contains silver ions. Typically, the preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. Salts of other organic acids or other organic materials, such as silver imidazolates, have been proposed. U.S. Pat. No. 4,260,677 discloses the use of complexes of inorganic or organic silver salts as non-photosensitive, reducible silver sources.
In both photographic and photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms (Ag.degree.). The imagewise distribution of these clusters is known in the art as a latent image. This latent image is generally not visible by ordinary means. Thus, the photosensitive emulsion must be further developed to produce a visible image. This is accomplished by the reduction of silver ions which are in catalytic proximity to silver halide grains bearing the clusters of silver atoms (i.e., the latent image). This produces a black-and-white image. In photographic elements, the silver halide is reduced to form the black-and-white image. In photothermographic elements, the light-insensitive silver source is reduced to form the visible black-and-white image while much of the silver halide remains as silver halide and is not reduced.
In photothermographic elements the reducing agent for the organic silver salt, often referred to as a "developer," may be any material, preferably any organic material, that can reduce silver ion to metallic silver and is preferably of relatively low activity until it is heated to a temperature above about 80.degree. C. At elevated temperatures, in the presence of the latent image, the silver ion of the non-photosensitive reducible silver source (e.g., silver behenate) is reduced by the reducing agent for silver ion. This produces a negative black-and-white image of elemental silver.
While conventional photographic developers such as methyl gallate, hydroquinone, substituted-hydroquinones, catechol, pyrogallol, ascorbic acid, and ascorbic acid derivatives are useful, they tend to result in very reactive photothermographic formulations and cause fog during preparation and coating of photothermographic elements. As a result, hindered phenol reducing agents have traditionally been preferred.
With the increased commercial availability of low-irradiance light sources such as light emitting diodes (LED), cathode ray tubes (CRT), and particularly semi-conductor laser diodes, as sources for output of electronically stored image data onto photosensitive films or paper, have come efforts to produce more highly sensitive photothermographic elements to match such exposure sources both in wavelength and sensitivity to light intensity. Such articles find particular utility in laser scanners.
Differences Between Photothermography and Photography
The imaging arts have long recognized that the field of photothermography is clearly distinct from that of photography. Photothermographic elements differ significantly from conventional silver halide photographic elements which require wet-processing.
In photothermographic imaging elements, a visible image is created by heat as a result of the reaction of a developer incorporated within the element. Heat is essential for development and temperatures of over 100.degree. C. are routinely required. In contrast, conventional wet-processed photographic imaging elements require processing in aqueous processing baths to provide a visible image (e.g., developing and fixing baths) and development is usually performed at a more moderate temperature (e.g., 30.degree.-50.degree. C.).
In photothermographic elements only a small amount of silver halide is used to capture light and a different form of silver (e.g., silver behenate) is used to generate the image with heat. Thus, the silver halide serves as a catalyst for the physical development of the non-photosensitive, reducible silver source. In contrast, conventional wet-processed black-and-white photographic elements use only one form of silver (e.g., silver halide): upon chemical development, the silver halide is itself converted to the silver image or upon physical development requires addition of an external silver source. Additionally, photothermographic elements require an amount of silver halide per unit area that is as little as one-hundredth of that used in conventional wet-processed silver halide.
Photothermographic systems employ a light-insensitive silver salt, such as silver behenate, which participates with the developer in developing the latent image. Chemically developed photographic systems do not employ a light-insensitive silver salt directly in the image-forming process. As a result, the image in photothermographic elements is produced primarily by reduction of the light-insensitive silver source (silver behenate) while the image in photographic black-and-white elements is produced primarily by the silver halide.
In photothermographic elements, all of the "chemistry" of the system is incorporated within the element itself. For example, photothermographic elements incorporate a developer (i.e., a reducing agent for the non-photosensitive reducible source of silver) within the element while conventional photographic elements do not. The incorporation of the developer into photothermographic elements can lead to increased formation of "fog" upon coating of photothermographic emulsions. Even in so-called instant photography, the developer chemistry is physically separated from the photosensitive silver halide until development is desired. Much effort has gone into the preparation and manufacture of photothermographic elements to minimize formation of fog upon coating, storage, and post-processing aging.
Similarly, in photothermographic elements, the unexposed silver halide inherently remains after development and the element must be stabilized against further development. In contrast, the silver halide is removed from photographic elements after development to prevent further imaging (i.e., the fixing step).
In photothermographic elements the binder is capable of wide variation and a number of binders are useful in preparing these elements. In contrast, photographic elements are limited almost exclusively to hydrophilic colloidal binders such as gelatin.
Because photothermographic elements require thermal processing, they pose different considerations and present distinctly different problems in manufacture and use. In addition, the effects of additives (e.g., stabilizers, antifoggants, speed enhancers, sensitizers, supersensitizers, etc.) which are intended to have a direct effect upon the imaging process can vary depending upon whether they have been incorporated in a photothermographic element or incorporated in a photographic element.
Because of these and other differences, additives which have one effect in conventional silver halide photography may behave quite differently in photothermographic elements where the underlying chemistry is so much more complex. For example, it is not uncommon for an antifoggant for a silver halide system to produce various types of fog when incorporated into photothermographic elements.
Distinctions between photothermographic and photographic elements are described in Imaging Processes and Materials (Neblette's Eighth Edition); J. Sturge et al. Ed; Van Nostrand Reinhold: New York, 1989, Chapter 9; in Unconventional Imaging Processes; E. Brinckman et al, Ed; The Focal Press: London and New York: 1978, pp. 74-75; and in C-f Zou, M. R. V Shayun, B. Levy, and N Serpone J. Imaging Sci. Technol. 1996, 40, 94-103.
In efforts to make more sensitive photothermographic materials, one of the most difficult parameters to maintain at a very low level is the various types of fog or D.sub.min. Fog is spurious image density which appears in non-imaged areas of the element after development and is often reported in sensitometric results as D.sub.min.
Without the ability to maintain speed, contrast, and resistance to fog, a commercially useful material is difficult to prepare. Various techniques have been employed to improve sensitivity and maintain resistance to fog.
U.S. Pat. No. 3,839,049 discloses a method of associating pre-formed silver halide grains with an organic silver salt dispersion. U.S. Pat. No. 4,161,408 (Winslow et al.) discloses a method of associating a silver halide emulsion with a silver soap by forming the silver soap in the presence of the silver halide emulsion. No sensitometric benefits for the process of this patent as compared to U.S. Pat. No. 3,839,049 are asserted. The process of U.S. Pat. No. 4,161,408 comprises adding silver halide grains with agitation to a dispersion of a long-chain fatty acid in water, with no alkali or metal salt of said fatty acid present while the acid is maintained above its melting point, then converting the acid to its ammonium or alkali metal salt, cooling the dispersion, and then converting the ammonium or alkali metal salt to a silver salt of the acid.
U.S. Pat. No. 4,212,937 describes the use of a nitrogen-containing organic base in combination with a halogen molecule or an organic haloamide to improve storage stability and sensitivity.
Japanese Patent Kokai 61-129 642, published Jun. 17, 1986, describes the use of halogenated compounds to reduce fog in color-forming photothermographic emulsions. These compounds include acetophenones such as phenyl-(.alpha.,.alpha.-dibromobenzyl)ketone.
U.S. Pat. No. 4,152,160 describes the use of carboxylic acids, such as benzoic acids and phthalic acids, in photothermographic elements. These acids are used as antifoggants.
U.S. Pat. No. 3,589,903 describes the use of small amounts of mercuric ion in photothermographic silver halide emulsions to improve speed and aging stability.
U.S. Pat. No. 4,784,939 describes the use of benzoic acid compounds of a defined formula to reduce fog and to improve the storage stability of silver halide photothermographic emulsions. The addition of halogen molecules to the emulsions are also described as improving fog and stability.
U.S. Pat. No. 5,064,753 discloses a thermally-developable, photographic material containing core-shell silver halide grains that contain a total of 4 to 40 mole % of silver iodide and which have a lower silver iodide content in the shell than in the core. Incorporating silver iodide into the silver halide crystal in amounts greater than 4 mole % is reported to result in increased photosensitivity and reduced D.sub.min. The silver halide itself is the primary component reduced to silver metal during development.
Japan Patent Kokai 63-300,234 discloses a heat-developable, photosensitive material containing a photosensitive silver halide, a reducing agent, and a binder. The photosensitive silver halide has a silver iodide content of 0.1.about.40 mole % and a core/shell grain structure. The photosensitive silver halide grains are further sensitized with gold. The material is reported to afford constructions with good sensitivity and low fog.
U.S. Pat. No. 5,434,043 discloses iridium doped pre-formed AgX grains to improve sensitivity and image quality of dry silver type photothermographic material.
The use of transition metal dopants to sensitize the silver halide emulsion and to reduce high-intensity reciprocity failure is known in conventional wet silver halide chemistry, particularly the use of group VIII transition metal ions. U.S. Pat. No. 5,051,344 and EP 743,554 both disclose photographic materials that contain iridium and iron as doping agents. The materials are described as having good speed and contrast properties.
As a photothermographic material is stored, or "ages", a number of difficulties can arise. As noted above, in contrast to conventional silver halide (AgX) chemistry, photothermographic materials contain all of the chemicals necessary for image development. During storage at ambient temperature and environmental humidity, slow chemical reactions between AgX/silver soap and surrounding developers/toners can occur which result in a gradual deterioration of sensitometry, such as fog formation in non-imaging areas and shifting of speed and contrast.
In addition to fog formation, photothermographic imaging materials also tend to slowly change speed and contrast upon shelf aging at ambient temperature and humidity. At elevated temperature and high humidity this process of deteriorating sensitometric properties is accelerated. Although stabilizers typically used in photothermographic material are effective to prevent fog formation, they are less effective in preventing speed and contrast changes, since this type of instability is usually associated with changes in the electronic and ionic properties of AgX micro-crystals during shelf storage. This typically represents only a small percentage of the total silver in the construction.
There is a need for a photothermographic emulsion that can be used to prepare photothermographic materials that can maintain speed and contrast properties, and resist fog, under shelf storage conditions.